How many people have ever lived on Earth?

As humanity welcomes its 7 billionth member this month, the Population Reference Bureau calculates our net population from 50,000 BC to today.

A HEAD OF THEIR TIME: Hundreds of unidentified human skulls, thought to pre-date the 13th century, are displayed at a crypt inside St. Leonard's church in Hythe, U.K. (Photo: ZUMA Press)

This time next week, there will be an unprecedented 7 billion people on Earth, according to U.N. estimates. That's a huge number for a single point in time, but how does it compare with our all-time population? Is it even possible to know how many humans have existed throughout history?

The nonprofit Population Reference Bureau has taken on this daunting challenge, and not just out of academic curiosity. As PRB demographer Carl Haub explains on the group's website, the question "is a perennial one among information calls to PRB," due partly to a myth from the 1970s claiming that 75 percent of all people who'd ever lived were alive at that moment.

The modern pace of human population growth has been extreme, as these charts show. But the idea that 4 billion people in 1974 — or even 7 billion in 2011 — represent three-quarters of every human in history is still far-fetched. Since it's not very scientific to call something wrong when you don't know what's right, though, Haub decided to find the answer himself.

This is a "semi-scientific" pursuit, he admits, since it not only requires population data from the dawn of man, but also a date for when that occurred. Scientists think the first Homo sapiens evolved some 200,000 years ago, but Haub starts with the origin of more modern humans about 50,000 years ago. Another caveat is that birth rates likely fluctuated wildly in the early days, when things like famine and predators may have pushed average life expectancy as low as 10 or 12 years, Haub says.

But rough population estimates do stretch back to at least 10,000 BC, so Haub used these and other factors to assign a constant birth rate to each historical period from 50,000 years ago to present. According to his calculations (which he dryly explains in the slow-paced video below), humanity has produced an astonishing 108 billion individual people over the past 50 millennia. That means today's 7 billion people represent about 6.5 percent of everyone who's ever lived.

The 75 percent myth is unfortunate, then, since it makes the actual percentage seem small by comparison. (If our current population really was 75 percent of Haub's all-time total, by the way, there would be 81 billion of us right now.) But 6.5 percent is a big deal when you have billions of something, especially tens or hundreds of billions. And it's well above 3.8 percent, which is what our mid-'70s population — 4 billion — represented of the roughly 105 billion people who had lived up to that point.

So as you contemplate this month's arrival of our 7 billionth fellow human, and the 108 billionth of all time, don't forget: We are the 6.5 percent. That may be less catchy than the populist motto of Occupy Wall Street, but it conveys a similar point. We're all in the same boat, and it could sink if we don't all figure out how to occupy it sustainably.